Add documentation about reactive viewmodels and listenable services.

[given8]

Code of Conduct

• I have read and agree to the Stacked Docs project's Code of Conduct

What document on stacked.filledstacks.com is affected?

https://stacked.filledstacks.com/docs/in-depth/services

What changes are you suggesting?

I would like to see the documentation about listenable services on the new website. It's currently in the old docs and it would be nice if all the stacked documentation is migrated to the website so it's all in one place.

Additional information

The old information is here: https://github.com/Stacked-Org/stacked/blob/master/README_old.md. Also note that some of the names of have been changed(deprecated) and you can only see this when you use the stated mixins and classes.

Pull Request

PR

[given8]

Here is the PR for adding the above